Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have also been proposed for use in vehicles as a replacement for internal combustion engines (ICE). A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen H2 at the anode generates hydrogen protons H+ and electrons e−. The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the membrane. In a second half-cell reaction, oxygen O2 at the cathode reacts with protons H+, and electrons e− are taken up to form water H2O.
The operation of ICE in cold weather has received intensive attention during the development of ICEs. Because fuel cells have become viable for widespread commercial use, cold weather performance characteristics of fuel cells have become more important. Vehicle manufacturers must address customer requirements and expectations to ensure that fuel cells are accepted. Some of these requirements include an affordable purchase price and operating costs, reliable and safe operation, traffic compatible performance such as acceleration and braking, range, payload, and ambient temperature tolerance.
The ambient temperature tolerance specification for vehicles typically includes temperatures between −40° C. to 52° C. Exemplary cold weather specifications for ICEs typically require that an engine must start within 30 seconds of cranking time with front and rear window defrosters operating. These specifications also require stable operation in low ambient temperatures such as −40° C. following a soak for eight hours at the low ambient temperature. Shortly after starting, the vehicle should be capable of driving away at varying rates of acceleration including wide-open throttle. Fuel cell vehicles will probably be required to meet similar cold weather specifications to meet consumer expectations.
Consumers expect automobiles to be operable in sub-freezing weather. Liquid and vapor water within the fuel cell system is a major concern for cold weather operation of the fuel cell. The cooling system typically employs de-ionized water. The fuel cell stack humidification systems and water generation at the cathode during operation generally ensures that water in a liquid or vapor state will exist in almost all parts of the fuel cell stack during dwell times. At one atmosphere and temperatures below 0° C., water freezes and may block the flow passages of the fuel cell stack. These blockages may expand and damage the fuel cell stack and/or render the cooling and humidification systems inoperable.
There are many approaches to prevent freezing within a fuel cell system during operation and/or as part of the start-up process. Passive freeze prevention involves several different approaches. One approach minimizes freezing by minimizing the use of water coolant. The water coolant can be replaced with a low freezing temperature heat transfer liquid that is not electrically conducting. Several major fuel cell suppliers are working in this area. Another option to limit the pure water coolant is to operate with separate stack and radiator coolant loops using a liquid-to-liquid heat exchanger. This approach, however, adds cost, weight and volume.
Another approach involves removing the water from the fuel cell system to prevent damage due to freezing while the fuel cell is not operating. The majority of the liquid water within the fuel cell stack can be removed at system shut-down using a gravity self-drain. Residual water within the stack can be removed by blowing dry, de-humidified air through the fuel cell stack just prior to system shut down. In these systems, water must be added before the fuel cell can be operated.
In other approaches, thermal insulation is placed around the entire fuel cell stack and the water reservoir to slow heat loss during operation and dwell times. The insulation is often integrated with the fuel cell stack casing. This approach increases stack volume and weight. Since fuel cell stack weight affects vehicle performance, installed power requirements, and cost, vehicle suppliers have an incentive to use other approaches that do not increase stack weight. Lowering fuel cell stack weight lowers thermal mass, decreases warm-up times but also decreases cool-down times.
Still other approaches involve operating the fuel cell to produce waste heat that is used to heat the fuel cell stack as part of the startup process. Advantages of this system include a relatively simple design that typically requires no system changes. Disadvantages of this system include the requirement for long warm-up times at sub-freezing temperatures because the fuel cell stack power is relatively low. This system also does not thaw ice in the anode and cathode flow channels directly. This system also does not address damage that may occur due to freezing when the fuel cell in the vehicle is not operating.
Other systems employ hot air from a compressor to melt ice in the anode and cathode flow channels and to warm the MEA during operation and/or as part of the start-up process. The hot air is typically greater than 90° C. and is available relatively quickly. Hot air warms all portions of the cathode. Disadvantages include the fact that not much heating power is derived from air (however, the thermal mass of the membrane electrode assembly is very low).
Other systems use hydrogen/air burners to warm stack coolant which, in turn, warms the fuel cell stack. Advantages of this heating system includes the generation of a large amount of high-quality heat. Waste heat can be used to warm the passenger compartment. Disadvantages include the decrease in, fuel economy and the requirement for the hydrogen/air burners that add weight, volume and cost to the fuel cell system.
Most of the preceding systems relate to the warming of the fuel cell stack at start-up or during vehicle operation but do not prevent damage that may be caused when the fuel cell stack remains in a dwell or off-mode at low ambient temperatures. Systems addressing the dwell-mode problem often include external plug-in resistive heaters. While consumers have accepted plug-in heaters for temperatures below −20° C., plug-in heater requirements for −20° C. to 0° C. are probably commercially unacceptable. Other solutions include a heated garage that is also probably commercially unacceptable.